It is possible to determine the amount of acidity of a solution by adding a caustic solution to it in a sufficient amount to neutralize the solution. The greater the amount of caustic solution required, the greater was the acidity of the original solution. A lesser amount of caustic solution needed is an indication that the original solution was less acid. The degree of acid or alkaline content of a solution is expressed as a pH number. pH is the negative logarithm of the hydrogen ion activity in gram equivalents per liter. pH 7 is considered neutral with pH numbers smaller than 7 indicating acidity and pH numbers larger than 7 indicating alkalinity. Various formulae have been devised to indicate how much more acid or caustic ingredients need be added to give the sample solution a desired greater acidity or caustic strength.
The foregoing problem and its solution is more difficult and is compounded when the sample solution includes three ingredients whose original concentrations were known but which become used up at different unknown rates during operation. Such is the case of a bath solution used in anodizing magnesium.
Magnesium is a light structural material that corrodes under ordinary atmospheric conditions and thus must be coated to protect it from corrosion. A preferred coating process prior to painting of the magnesium is anodizing. Chemical Treatment No. 17 of Dow Chemical Company is a typical anodizing bath consisting of ammonium acid fluoride or ammonium bifluoride (NH.sub.4 HF.sub.2), sodium dichromate (Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O) and phosphoric acid (H.sub.3 PO.sub.4). At this bath is used in the anodizing process, the proportionate amounts of these chemicals will vary as the ingredients are used up at different rates. With a resulting improperly mixed bath, the magnesium part doesn't have the adequate protective build-up that it should; it displays rejectionable colors and other processing problems which result in an unacceptable anodized part. For this reason it is important to maintain the proper mix of the anodizing solution.
Current analytical procedures for determining the content of a magnesium anodizing bath are set forth in a Dow Chemical Company Instruction Form No. 147-39-70 entitled, Product and Process Data--Magnesium--Chemical Treatment No. 17, which has been available since at least 1973. These procedures are also set forth in MIL-M-45202B. Briefly, chromate is reduced with excess ferrous sulfate. The excess ferrous iron is titrated with potassium dichromate. An alternate procedure for determining chromium is by adding hydrochloric acid (HCl) and potassium iodide (KI) to the sample to liberate free iodine. Iodine is titrated with sodium thiosulfate with a starch indicator. This is known as the thiosulfate method. Phosphorus is precipitated as ammonium phosphomolybdate, the precipitate is dissolved in an excess of standard base, and the excess base is titrated with standard acid to the phenolphthalein end point. Interference from fluorides is prevented by adding boric acid to form harmless fluoboric acid. The colored chromate ion is precipitated as silver chromate in an alkaline solution. The solution is filtered and the fluoride ion in the filtrate is determined by titration with thorium nitrate using alizarin sulfonate as an indicator.
Current analytical procedures involve complex precipitation techniques which are time consuming, requiring approximately twenty hours per analysis. The new control procedure of the present invention is reliable yet requires only one hour because the time required for fluoride and phosphorus analysis is greatly reduced.